Basically follow this link: https://support.apple.com/en-us/HT201065
When you get Step 7 from the link, come back here and follow the below steps:
1. Shutdown your mac
2. Enter Internet Recovery Mode by starting your mac book, then immediately hold down the 3 keys together “Command-Option-R” and release the keys when you see the Apple logo.
3. Choose your wifi network from the list to get connected to the internet. Note: If you have trouble with WiFi, plug in a Ethernet cable (wired)
4. It’ll start the recover, then it’ll take you to a screen with a few options. Select Reinstall OS….